Light emitting element

ABSTRACT

A light emitting element includes, successively from a lower side to an upper side, a first light emitting part having a first active layer, a tunnel junction part, and a second light emitting part having a second active layer. The first active layer includes a plurality of first well layers, and a first barrier layer positioned between two adjacent first well layers among the first well layers. The second active layer includes a plurality of second well layers, and a second barrier layer positioned between two adjacent second well layers among the second well layers. The second barrier layer is a nitride semiconductor layer containing an n-type impurity and gallium, and has an n-type impurity concentration higher than that of the first barrier layer. An n-type impurity concentration peak in the second barrier layer is located on a first light emitting part side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to Japanese PatentApplication No. 2021-189304, filed on Nov. 22, 2021, the entire contentsof which are incorporated herein by reference.

FIELD

The present disclosure relates to a light emitting element.

BACKGROUND

Japanese Patent Publication No. 2017-157667, for example, discloses alight emitting element comprising nitride semiconductor layers thatinclude a tunnel junction layer.

SUMMARY

There is a desire to improve the emission efficiency of such a lightemitting element. An object of certain embodiments of the presentdisclosure is to provide a light emitting element that can increase theemission efficiency.

According to one embodiment of the present disclosure, a light emittingelement has, successively from the lower side to the upper side, a firstlight emitting part having a first active layer, a tunnel junction part,and a second light emitting part having a second active layer. The firstactive layer has a plurality of first well layers and a first barrierlayer positioned between two adjacent first well layers among the firstwell layers and having a wider band gap than the band gaps of the firstwell layers. The second active layer has a plurality of second welllayers and a second barrier layer positioned between two adjacent secondwell layers among the second well layers and having a wider band gapthan the band gaps of the second well layers, wherein the second barrierlayer is a nitride semiconductor layer containing an n-type impurity andgallium, and having a higher n-type impurity concentration than then-type impurity concentration of the first barrier layer, and the n-typeimpurity concentration peak in the second barrier layer is located onthe first light emitting part side.

According to certain embodiments of the present disclosure, a lightemitting element with improved emission efficiency can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a light emitting element accordingto an embodiment.

FIG. 2 is a cross-sectional view of a first active layer of theembodiment.

FIG. 3 is a cross-sectional view of a second active layer of theembodiment.

FIG. 4 is a cross-sectional view of a portion of a second active layerin a variation of the embodiment.

FIG. 5A is a graph showing the forward voltage measurement results ofthe light emitting elements according to the embodiment.

FIG. 5B is a graph showing the light output measurement results of thelight emitting elements according to the embodiment.

FIG. 6A is a graph showing the forward voltage measurement results ofthe light emitting elements according to the embodiment.

FIG. 6B is a graph showing the light output measurement results of thelight emitting elements according to the embodiment.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure will be explained belowwith reference to the accompanying drawings. In the drawings, the sameconstituents are denoted by the same reference numerals. Each drawing isa schematic illustration of an embodiment. As such, the scale, spacing,or positional relationship of members might be exaggerated, or a portionof a member omitted. Furthermore, as a cross-sectional view, an end faceview showing a cut cross section might be used.

FIG. 1 is a cross-sectional view of a light emitting element 1 accordingto an embodiment.

The light emitting element 1 has a substrate 10, a semiconductor stackstructure 20, a p-side electrode 11, and an n-side electrode 12.

A substrate 10 supports a semiconductor stack structure 20. For thematerial for the substrate 10, for example, sapphire, silicon, SiC, GaN,or the like can be used. In the case of using a sapphire substrate asthe substrate 10, the semiconductor stack structure 20 is deposited onthe c-plane of the sapphire substrate.

A semiconductor stack structure 20 is a stacked structure in which aplurality of semiconductor layers made of nitride semiconductors arestacked. Nitride semiconductors can include all semiconductors obtainedby varying the composition ratio x and y within their ranges in thechemical formula In_(x)Al_(y)Ga_(1−x−y)N (0≤x≤1, 0≤y≤1, x+y≤1). Forexample, the semiconductor stack structure 20 is formed by epitaxiallygrowing semiconductors on the substrate 10.

In the present specification, the lower side is closer to the substrate10 relative to the upper side. The semiconductor stack structure 20includes, successively from the lower side to the upper side, a firstlight emitting part 21, a tunnel junction part 30, and a second lightemitting part 22.

A first light emitting part 21 has an n-side nitride semiconductor layer41 positioned on the substrate 10, a first superlattice layer 50positioned on the n-side nitride semiconductor layer 41, a first activelayer 60 positioned on the first superlattice layer 50, and a firstp-side nitride semiconductor layer 42 positioned on the first activelayer 60.

A second light emitting part 22 has a second superlattice layer 70positioned on a tunnel junction part 30, a second active layer 80positioned on the second super lattice layer 70, and a second p-sidenitride semiconductor layer 43 positioned on the second active layer 80.

An n-side nitride semiconductor layer 41 has an n-type layer containingan n-type impurity. The n-type layer contains, for example, silicon (Si)as the n-type impurity. Alternatively, the n-type layer may containgermanium (Ge) as the n-type impurity. The n-side semiconductor layer 41is sufficient if it has the function of supplying electrons, and mayinclude an undoped layer formed without intentionally doping with ann-type or p-type impurity. The undoped layer in the case of beingadjacent to a layer intentionally doped with an n-type impurity and/or ap-type impurity might contain the n-type impurity and/or the p-typeimpurity through diffusion from the adjacent layer.

A first p-side nitride semiconductor layer 42 and a second p-sidenitride semiconductor layer 43 each have a p-type layer containing ap-type impurity. Such a p-type layer contains, for example, magnesium(Mg) as the p-type impurity. The first p-side nitride semiconductorlayer 42 and the second p-side nitride semiconductor layer 43 aresufficient if they have the function of supplying positive holes, andmay include an undoped layer.

A first active layer 60 and a second active layer 80 have amulti-quantum well structure that includes a plurality of well layersand a plurality of barrier layers as described below. The first activelayer 60 and the second active layer 80 can emit blue light or greenlight, for example. The peak emission wavelength of blue light is 430 nmto 490 nm. The peak emission wavelength of green light is 500 nm to 540nm. The peak emission wavelength of the emitted light from the firstactive layer 60 may be the same as or different from that from thesecond active layer 80. The first active layer 60 and the second activelayer 80 can emit light having a shorter peak emission wavelength thanthat of blue light or light having a longer peak emission wavelengththan that of green light.

A tunnel junction part 30 includes a nitride semiconductor layer. Thetunnel junction part 30 forms a tunnel junction with the first p-sidenitride semiconductor layer 42. The tunnel junction part 30 has at leastone semiconductor layer among p-type and n-type layers. A p-type layeris disposed in contact with the upper face of the first p-side nitridesemiconductor layer 42, and contains, for example, magnesium as a p-typeimpurity. If a p-type layer is disposed, an n-type layer is disposed onthe upper face of the p-type layer. If no p-type layer is disposed, then-type layer is disposed in contact with the upper face of the firstp-side nitride semiconductor layer 42. The n-type layer contains, forexample, silicon as an n-type impurity.

A first superlattice layer 50 is positioned between the n-side nitridesemiconductor layer 41 and the first active layer 60. A secondsuperlattice layer 70 is positioned between the tunnel junction part 30and the second active layer 80. Providing a first superlattice layer 50and a second superlattice layer 70 can reduce the lattice mismatchbetween the substrate 10 and the semiconductor stack structure 20,thereby reducing crystal defects in the semiconductor stack structure20.

The first superlattice layer 50 and the second superlattice layer 70each have a plurality of first nitride semiconductor layers and aplurality of second nitride semiconductor layers. The first superlatticelayer 50 and the second superlattice layer 70 can each have 15 to 25pairs of a first nitride semiconductor layer and a second nitridesemiconductor layer. The first superlattice layer 50 and the secondsuperlattice layer 70 can each have, for example, twenty first nitridesemiconductor layers and twenty second nitride semiconductor layers. Ineach of the first superlattice layer 50 and the second superlatticelayer 70, a second nitride semiconductor layer is in the lowest position(the lowermost layer), and a first nitride semiconductor layer is in thehighest position (the uppermost layer). From the second nitridesemiconductor layer, the lowermost layer, to the first nitridesemiconductor layer, the uppermost layer, the second nitridesemiconductor layers and the first nitride semiconductor layers areformed alternately.

The composition of a first nitride semiconductor layer differs from thecomposition of a second nitride semiconductor layer. The first nitridesemiconductor layers in the first superlattice layer 50 are, forexample, undoped InGaN layers. The In composition ratio in the InGaNlayers can be set in a range of 5% to 10%. The second nitridesemiconductor layers in the first superlattice layer 50 are, forexample, undoped GaN layers. The first nitride semiconductor layers inthe second superlattice layer 70 are, for example, silicon-doped n-typeInGaN layers. The In composition ratio in the InGaN layers can be set ina range of 5% to 10%. The second nitride semiconductor layers in thesecond superlattice layer 70 are, for example, silicon-doped n-type GaNlayers. The n-type impurity concentration of the first nitridesemiconductor layers and the second nitride semiconductor layers of thesecond superlattice layer 70 can be set, for example, in a range of1×10¹⁷ cm⁻³ to 1×10¹⁸ cm⁻³. The n-type impurity concentration of each ofthe first nitride semiconductor layers and the second nitridesemiconductor layers refers to the highest n-type impurity concentrationamong all concentrations in the respective first nitride semiconductorlayers and the second nitride semiconductor layers.

In the first superlattice layer 50 and the second superlattice layer 70,the thicknesses of the first nitride semiconductor layers are smallerthan the thicknesses of the second nitride semiconductor layers. Forexample, the thicknesses of the first nitride semiconductor layers canbe set in a range of 0.5 nm to 1.5 nm. For example, the thicknesses thesecond nitride semiconductor layers can be set in a range of 1.5 nm to 3nm.

The n-side nitride semiconductor layer 41 has an n-side contact face 41a on which no semiconductor layer is disposed. An n-side electrode 12 isdisposed on the n-side contact face 41 a. The n-side electrode 12 iselectrically connected to the n-side nitride semiconductor layer 41.

A p-side electrode 11 is disposed on the upper face of the second p-sidenitride semiconductor layer 43. The p-side electrode 11 is electricallyconnected to the second p-side nitride semiconductor layer 43.

A forward voltage is applied across the p-side electrode 11 and then-side electrode 12. At this time, a forward voltage is applied acrossthe second p-side nitride semiconductor layer 43 of the second lightemitting part 22 and the n-side nitride semiconductor layer 41 of thefirst light emitting part 21, supplying positive holes and electrons tothe first active layer 60 and the second active layer 80 therebyallowing the first active layer 60 and the second active layer 80 toemit light.

According to a light emitting element 1 of this embodiment, in which asecond active layer 80 is provided above a first active layer 60, theper unit area output can be increased as compared to a light emittingelement having a single active layer.

When a forward voltage is applied across the p-side electrode 11 and then-side electrode 12, a reverse voltage would apply to the tunneljunction formed by the tunnel junction part 30 and the first p-sidenitride semiconductor layer 42. Accordingly, allowing the p-type layerand the n-type layer that form the tunnel junction to respectively havehigh p-type and n-type impurity concentrations can narrow the width ofthe depletion layer formed by the junction of the tunnel junction part30 and the first p-side nitride semiconductor layer 42. This allows forthe tunneling of the electrons present in the valence band in the p-typelayer to the conduction band of the n-type layer to thereby facilitatethe electric current flow to the tunnel junction part 30.

The first active layer 60 and the second active layer 80 will beexplained in detail below.

First Active Layer

As shown in FIG. 2 , the first active layer 60 has a plurality of firstwell layers 61 and at least one first barrier layer 65. The first activelayer 60 has, for example, three or more first well layers 61 and two ormore first barrier layers 65. The first active layer 60 can have, forexample, seven first well layers 61 and six first barrier layers 65.Each first barrier layer 65 is positioned between two adjacent firstwell layers 61 among the first well layers 61. The active layer 60 canfurther have a fourth barrier layer 63 in the lowest position in thefirst active layer 60 and a fifth barrier layer 64 in the highestposition in the first active layer 60. A first well layer 61 is disposedbetween the fourth barrier layer 63 and the first barrier layer 65 thathas the lowest position among the first barrier layers 65. A first welllayer 61 is disposed between the fifth barrier layer 64 and the firstbarrier layer 65 that has the highest position among the first barrierlayers 65. Between the fourth barrier layer 63 and the fifth barrierlayer 64, the first well layers 61 and the first barrier layers 65 arealternately provided.

The band gaps of the first barrier layers 65, the fourth barrier layer63, and the fifth barrier layer 64 are wider than the band gaps of thefirst well layers 61. The first well layers 61, the first barrier layers65, the fourth barrier layer 63, and the fifth barrier layer 64 arenitride semiconductor layers containing gallium. The first well layers61 contain gallium and indium. For example, the first well layers 61 areundoped InGaN layers. In a case in which the first well layers 61 areInGaN layers, the In composition ratio can be set in a range of 12% to18%. The first well layers 61 may contain aluminum. The first barrierlayers 65 and the fifth barrier layer 64 are, for example, undoped GaNlayers. The n-type impurity concentration of the first barrier layers 65can be set, for example, in a range of 1×10¹⁷ cm⁻³. The fourth barrierlayer 63 is, for example, an n-type GaN layer. The fourth barrier layer63 contains silicon or germanium as the n-type impurity.

The thicknesses the first barrier layers 65 and the fifth barrier layer64 are larger than the thicknesses of the first well layer 61. Forexample, the thicknesses of the first well layers 61 can be set in arange of 2.5 nm to 4 nm. For example, the thicknesses of the firstbarrier layers 65 and the fifth barrier layer 64 can be set in a rangeof 3 nm to 5 nm. The thickness of the fourth barrier layer 63 can be setin a range of 3 nm to 5 nm.

Second Active Layer

As shown in FIG. 3 , the second active layer 80 has a plurality ofsecond well layers 81 and at least one second barrier layer 82. Thesecond active layer 80 has, for example, three or more second welllayers 81 and two or more second barrier layers 82. The second activelayer 80 can have, for example, seven second well layers 81 and sixsecond barrier layers 82. Each second barrier layer 82 is positionedbetween two adjacent second well layers 81 among the second well layers81.

Furthermore, the second active layer 80 can have a sixth barrier layer83 in the lowest position in the second active layer 80 and a thirdbarrier layer 84 in the highest position in the second active layer 80.A second well layer 81 is disposed between the sixth barrier layer 83and the second barrier layer 82 that has the lowest position among thesecond barrier layers 82. A second well layer 81 is disposed between thethird barrier layer 84 and the second barrier layer 82 that has thehighest position among the second barrier layers 82. Between the sixthbarrier layer 83 and the third barrier layer 84, the second well layers81 and the second barrier layers 82 are alternately provided.

The band gaps of the second barrier layers 82, the sixth barrier layer83, and the third barrier layer 84 are wider than the band gaps of thesecond well layers 81. The second well layers 81, the second barrierlayers 82, the sixth barrier layer 83 and the third barrier layer 84 arenitride semiconductor layers containing gallium.

The second well layers 81 can contain gallium and indium. The secondwell layers 81 are, for example, undoped InGaN layers. In a case inwhich the second well layers 81 are InGaN layers, the In compositionratio can be set in a range of 12% to 18%. The second well layers 81 maycontain aluminum.

The second barrier layers 82 contain an n-type impurity and gallium. Thesecond barrier layers 82 contain, for example, silicon or germanium asthe n-type impurity. The n-type impurity concentration of the secondbarrier layers 82 is higher than the n-type impurity concentration ofthe first barrier layers 65. The n-type impurity concentration peak inat least one of the second barrier layers 82 is located on the firstlight emitting part 21 side. The n-type impurity concentration peak ispreferably located on the first light emitting part 21 side in allsecond barrier layers 82.

At least one of the second barrier layers 82 has a first layer 82 a anda second layer 82 b in that order from the first light emitting part 21side. Every one of the second barrier layers 82 a preferably has a firstlayer 82 a and a second layer 82 b. A second barrier layer 82 thatincludes both a first layer 82 a and second layer 82 b can be formed byforming a first layer 82 a, followed by forming a second layer 82 b onthe first layer 82 a. For example, a second barrier layer 82 can beformed by forming an n-type GaN layer as a first layer 82 a, followed byforming on the first layer 82 a an undoped GaN layer as a second layer82 b.

The n-type impurity concentration of a first layer 82 a is higher thanthe n-type impurity concentration of a second layer 82 b. The n-typeimpurity concentration peak in a second barrier layer 82 is positionedin the first layer 82 a. The n-type impurity concentration of a firstlayer 82 a is lower than the n-type impurity concentration of the tunneljunction part 30. The n-type impurity concentration of a first layer 82a is lower than the n-type impurity concentration of the secondsuperlattice layer 70. The n-type impurity concentration of a firstlayer 82 a can be set, for example, in a range of 2×10¹⁸ cm⁻³ to 5×10¹⁸cm⁻³. The n-type impurity concentration of the tunnel junction part 30can be set in a range of 1×10²⁰ cm⁻³ to 5×10²¹ cm⁻³. The n-type impurityconcentration of the second superlattice layer 70 can be set in a rangeof 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³. Furthermore, the n-type impurityconcentration of a first layer 82 n is preferably higher than the p-typeimpurity concentration of the second barrier layer 82. This can preventthe second active layer 80 of the second light emitting part 22described below from turning into a p-type layer. The n-type impurityconcentration of a first layer 82 a refers to the highest n-typeconcentration in the first layer 82 a. The p-type impurity concentrationof a second barrier layer 82 refers to the highest p-type concentrationin the second barrier layer 82.

The n-type impurity concentration of the third barrier layer 84 is lowerthan the n-type impurity concentration in the second barrier layers 82.The third barrier layer 84 is, for example, an undoped GaN layer. Thesixth barrier layer 83 is, for example, an n-type GaN layer. The sixthbarrier layer 83 contains silicon or germanium as an n-type impurity.The n-type impurity concentration of the third barrier layer 84 refersto the highest n-type impurity concentration in the third barrier layer84.

The thicknesses of the second barrier layers 82 and the third barrierlayer 84 are larger than the thicknesses of the second well layers 81.For example, the thicknesses of the second well layers 81 can be set ina range of 2.5 nm to 4 nm. For example, the thicknesses of the secondbarrier layers 82 and the third barrier layer can be set in a range of 3nm to 5 nm. The thickness of the sixth barrier layer 83 can be set in arange of 3 nm to 5 nm.

It is preferable to set the thickness of a first layer 82 a in eachsecond barrier layer 82 as 10% to 50%, more preferably 10% to 25% of thethickness of the second barrier layer 82. Setting the thickness of afirst layer 82 a in each second barrier layer 82 as 10% to 50% of thethickness of the second barrier layer 82 can readily increase the lightoutput while reducing the forward voltage of the light emitting element.The thickness of a first layer 82 a is preferably set to fall within the0.5 nm to 2 nm range, for example, more preferably the 0.5 nm to 1 nmrange.

In a light emitting element in which a second light emitting part isformed on a first light emitting part via a tunnel junction part, forexample, the p-type impurity (e.g., magnesium) contained in the firstp-side nitride semiconductor layer might diffuse into the second lightemitting part during the formation of the second light emitting part onthe tunnel junction part. If the p-type impurity is diffused into thesecond light emitting part, the second active layer of the second lightemitting part would unintentionally turn into p-type to thereby increasethe forward voltage of the light emitting element. This, as a result,reduces the emission efficiency of the light emitting element.

According to this embodiment, the second barrier layers 82 in the secondactive layer 80 of the second light emitting part 22 contain a higherconcentration n-type impurity than the first barrier layers 65 of thefirst active layer 60 of the first light emitting part, and the n-typeimpurity concentration peaks in the second barrier layers 82 are locatedon the first light emitting part 21 side. This can reduce the turning ofthe second active layer 80 into p-type attributable to the diffusion ofthe p-type impurity from the first p-side nitride semiconductor layer42. This, as a result, can increase the electron injection efficiencyinto the second light emitting part 22 thereby increasing the internalquantum efficiency. This can reduce the forward voltage therebyincreasing the emission efficiency.

Making the n-type impurity concentration of the first barrier layers 65in the first active layer 60 relatively high in the first light emittingpart 21 does not tend to lead to the improvement in the emissionefficiency of the light emitting element 1. This is because the p-typeimpurity contained in the first p-side nitride semiconductor layer 42readily diffuses into the second light emitting part 22, i.e., it isunlikely for the first active layer 60 in the first light emitting part21 to unintentionally turn into p-type.

Furthermore, if the n-type impurity concentration peak in a secondbarrier layer 82 is located on the second p-side nitride semiconductorlayer 43 side instead of being on the first light emitting part 21 side,the n-type impurity might be unintentionally mixed into the second welllayers 81 formed on the second barrier layers 82 to degrade thecrystalline quality. According to this embodiment, the n-type impurityconcentration peak in each second barrier layer 82 is positioned on thefirst light emitting part 21 side to thereby maintain the crystallinequality of the second well layers 81 while preventing the second activelayer 80 from turning into p-type. As a result, a light emitting elementwith improved emission efficiency can be provided.

Next, the forward voltage and light output measurement results of thesamples of the light emitting element 1 according to this embodimentwill be explained.

Each of the light emitting element 1 samples produced had theconstituents described below.

The substrate 10 was a sapphire substrate.

The n-side nitride semiconductor layer 41 contained silicon as a n-typeimpurity. The silicon concentration of the n-side nitride semiconductorlayer 41 was about 1×10¹⁹ cm⁻³. The silicon concentration of the n-sidenitride semiconductor layer 41 refers to the highest siliconconcentration in the n-side nitride semiconductor layer 41. Thethickness of the n-side nitride semiconductor layer 41 was about 5 μm.

The first superlattice layer 50 had twenty undoped InGaN layers andtwenty undoped GaN layers. In the first superlattice layer 50, a GaNlayer was in the lowest position (the lowermost layer), and an InGaNlayer was in the highest position (the uppermost layer). GaN layers andInGaN layers were alternately disposed from the GaN layer that was thelowermost layer to the InGaN layer that was the uppermost layer. The Incomposition ratio of each InGaN layer was about 7%. The thickness ofeach InGaN layer was about 1 nm. The thickness of each GaN layer wasabout 2 nm.

The first active layer 60 had seven first well layers 61 and six firstbarrier layers 65. The first active layer 60 further had a fourthbarrier layer 63 positioned lowest in the first active layer 60, and afifth barrier layer 64 positioned highest in the first active layer 60.The first well layers 61 were undoped InGaN layers. The In compositionratio of the first well layers 61 was about 15%. The thickness of eachfirst well layer 61 was about 3.5 nm. The first barrier layers 65 wereundoped GaN layers. The thickness of each first barrier layer 65 wasabout 4 nm. The fourth barrier layer 63 included, successively from thefirst superlattice layer 50 side, a silicon-doped InGaN layer and anundoped GaN layer. The thickness of the fourth barrier layer 63 wasabout 3.5 nm. The fifth barrier layer 64 was an undoped GaN layer. Thethickness of the fifth barrier layer 64 was about 4 nm.

The first p-side nitride semiconductor layer 42 contained magnesium as ap-type impurity. The magnesium concentration of the first p-side nitridesemiconductor 42 was about 5×10²⁰ cm⁻³. The magnesium concentration ofthe first p-side nitride semiconductor 42 refers to the highestmagnesium concentration in the first p-side nitride semiconductor 42.The thickness of the first p-side nitride semiconductor 42 was about 80nm.

The tunnel junction part 30 included, successively from the first p-sidenitride semiconductor 42 side, a magnesium-doped p-type GaN layer and asilicon-doped n-type GaN layer. The silicon concentration of the n-typeGaN layer was about 5×10²⁰ cm⁻³. The thickness of the n-type GaN layerwas about 150 nm.

The second superlattice layer 70 had twenty silicon-doped InGaN layersand twenty silicon-doped GaN layers. In the second superlattice layer70, a GaN layer was in the lowest position (the lowermost layer) and anInGaN layer was in the highest position (the uppermost layer). GaNlayers and InGaN layers were alternately disposed from the GaN layerthat was the lowermost layer to the InGaN layer that was the uppermostlayer. The In composition ratio of each InGaN layer was about 7%. Thethickness of each InGaN layer was about 1 nm. The thickness of each GaNlayer was about 2 nm. The silicon concentration of the InGaN and GaNlayers was about 1×10¹⁹ cm⁻³.

The second active layer 80 had seven second well layers 81 and sixsecond barrier layers 82. The second active layer 80 further had a sixthbarrier layer 83 that was positioned lowest in the second active layer80, and a third barrier layer 84 that was positioned highest in thesecond active layer 80. The second well layers 81 were undoped InGaNlayers. The In composition ratio of the second well layers 81 was about15%. The thickness of each second well layer 81 was about 3.5 nm. Eachsecond barrier layer 82 had a first layer 82 a and a second layer 82 b.The first layers 82 a were silicon-doped GaN layers. The second layers82 b were undoped GaN layers. The sixth barrier layer 83 included,successively from the second super lattice layer 70 side, asilicon-doped InGaN layer and a undoped GaN layer. The thickness of thesixth barrier layer 83 was about 3.5 nm. The third barrier layer 84 wasan undoped GaN layer. The thickness of the third barrier layer 84 wasabout 4 nm.

The second p-side nitride semiconductor layer 43 contained magnesium asa p-type impurity. The magnesium concentration of the second p-sidenitride semiconductor layer 43 was about 5×10²⁰ cm⁻³. The magnesiumconcentration of the second p-side nitride semiconductor layer 43 refersto the highest magnesium concentration in the second p-side nitridesemiconductor layer 43. The thickness of the second p-side nitridesemiconductor layer 43 was about 100 nm.

FIG. 5A is a graph showing the forward voltage measurement results ofthe light emitting element 1 samples when the forward current appliedwas 120 mA. The silicon concentration of the first layers 82 a in eachof the light emitting element 1 samples was about 1×10¹⁸ cm⁻³. In FIG.5A, the horizontal axis represents the thickness (nm) of the firstlayers 82 a in which the silicon concentration peaks in the respectivesecond barrier layers 82 were located. FIG. 5A and FIG. 5B show themeasurement results in cases in which the thickness of the first layers82 a was 0.5 nm, 1 nm, 2 nm, 3 nm, and 4 nm. The thickness of the firstlayers 82 a being zero means that the second barrier layers 82 did notinclude any first layer 82 a, i.e., they were undoped GaN layers.Furthermore, the thickness of the second barrier layers 82 in eachsample was 4 nm. The thickness of the first layers 82 a being 4 nm meansthat the second barrier layers 82 were silicon-doped n-type first layers82 a. In FIG. 5A, the vertical axis represents the amount of change inthe forward voltage (V) as compared to the forward voltage (0.00 V)assumed in a case in which the thickness of the first layers 82 a waszero.

FIG. 5B is a graph showing the light output measurement results of thelight emitting element 1 samples when the forward current applied was120 mA. The silicon concentration of the first layers 82 a in each ofthe light emitting element 1 samples was about 1×10¹⁸ cm⁻³. In FIG. 5B,the horizontal axis represents the thickness (nm) of the first layers 82a in the second barrier layers 82. In FIG. 5B, the vertical axisrepresents the relative light output value as compared to the lightoutput (1.00) assumed in a case in which the thickness of the firstlayers 82 a was zero.

As shown by the results in FIG. 5A and FIG. 5B, the forward voltagedeclined in the light emitting element 1 samples according to theembodiment in which the thickness of the first layers 82 a was 0.5 nm to4 nm as compared to a case in which the thickness of the first layers 82a was zero. Furthermore, the samples in which the first layers 82 a was0.5 nm to 2 nm in thickness achieved light outputs equivalent to orhigher than a case in which the thickness of the first layers 82 a waszero while reducing the forward voltage as compared to a case in whichthe thickness of the first layers 82 a was zero. Accordingly, thepreferable thickness range for the first layers 82 a in the secondbarrier layers 82 in achieving an equivalent or higher light outputwhile reducing the forward voltage is 0.5 nm to 2 nm. The morepreferable thickness range for the first layers 82 a, to furtherincrease the light output while reducing the forward voltage as comparedto a case in which the thickness of the first layers 82 a is zero, is0.5 nm to 1 nm.

FIG. 6A is a graph showing the forward voltage measurement results ofthe light emitting element 1 samples with varied silicon concentrationsfor the first layers 82 a when the forward current applied was 120 mA.The thickness of the second barrier layers 82 in each sample was about 4nm, and the thickness of the first layers 82 a in the second barrierlayers 82 was about 0.5 nm. In FIG. 6A, the horizontal axis representsthe silicon concentration (cm⁻³) of the first layers 82 a. FIG. 6A andFIG. 6B show the measurement results in the cases in which the siliconconcentration of the first layers 82 a was 2×10¹⁸ cm⁻³, 5×10¹⁸ cm⁻³, and1×10¹⁹ cm⁻³. The silicon concentration being zero means that the secondbarrier layers 82 did not include any first layer 82 a, i.e., they wereundoped GaN layers. In FIG. 6A, the vertical axis represents the amountof change in the forward voltage (V) as compared to the forward voltage(0.00 V) assumed in a case in which the silicon concentration was zero.

FIG. 6B is a graph showing the light output measurement results of thelight emitting element 1 samples with varied silicon concentrations forthe first layers 82 a when the forward current applied was 120 mA. Thethickness of the second barrier layers 82 in each sample was about 4 nm,and the thickness of the first layers 82 a in the second barrier layers82 was about 0.5 nm. In FIG. 6B, the horizontal axis represents thesilicon concentration (cm⁻³) of the first layers 82 a. In FIG. 6B, thevertical axis represents the relative light output value as compared tothe light output (1.00) assumed in a case in which the siliconconcentration was zero.

As shown by the results in FIG. 6A and FIG. 6B, the forward voltagedeclined in the light emitting element 1 samples according to theembodiment when the silicon concentration of the first layers 82 a was2×10¹⁸ cm⁻³ to 1×10¹⁹ cm⁻³ as compared to the case of zero siliconconcentration. Furthermore, the samples in which the siliconconcentration of the first layers 82 a was 2×10¹⁸ cm⁻³ to 5×10¹⁸ cm⁻³achieved light outputs equivalent to or higher than that in the case ofzero silicon concentration while reducing the forward voltage ascompared to case of zero silicon concentration. Accordingly, thepreferable silicon concentration range for the first layers 82 a is2×10¹⁸ cm⁻³ to 5×10¹⁸ cm⁻³.

As shown in FIG. 4 , each second barrier layer 82 can further have athird layer 82 c that is positioned closer to the first light emittingpart 21 than the first layer 82 a is. FIG. 4 is a cross-sectional viewof a portion of the second active layer 80 according to a variation ofthe embodiment. FIG. 4 shows two adjacent second well layers 81 and asecond barrier layer 82 positioned between the second well layers 81.

A second barrier layer 82 can be formed by forming a second well layer81, followed by forming a third layer 82 c on the second well layer 81,forming a first layer 82 a on the third layer 82 c, and forming a secondlayer 82 b on the first layer 82 a. For example, a second barrier layer82 is formed by forming an undoped GaN layer as a third layer 82 c,forming an n-type GaN layer as a first layer 82 a on the third layer 82c, and forming an undoped GaN layer as a second layer 82 b on the firstlayer 82 a. Providing such a third layer 82 c can further improve thecrystalline quality of the first layer 82 a as well as the crystallinequality of the second well layer 81 positioned above the first layer 82a as compared to the case in which the first layer 82 a is formed incontact with the second swell layer 81 formed thereunder.

The n-type impurity concentration of the second layer 82 b and then-type impurity concentration of the third layer 82 c are lower than then-type impurity concentration of the first layer 82 a. This can furtherimprove the crystalline quality of the first layer 82 a. The third layer82 c is thinner than the second layer 82 b. The thickness of the thirdlayer 82 c can be set in a range of 0.1 nm to 1 nm.

In the foregoing, certain embodiments of the present disclosure havebeen explained with reference to specific examples. The presentinvention, however, is not limited to these specific examples. All formsimplementable by a person skilled in the art by suitably making designchanges based on any of the embodiments of the present disclosuredescribed above also fall within the scope of the present invention solong as they encompass the subject matter of the present invention.Furthermore, various modifications and alterations within the spirit ofthe present disclosure that could have been made by a person skilled inthe art also fall within the scope of the present invention.

What is claimed is:
 1. A light emitting element comprising: successivelyfrom a lower side to an upper side: a first light emitting part having afirst active layer; a tunnel junction part; and a second light emittingpart having a second active layer; wherein: the first active layercomprises: a plurality of first well layers, and a first barrier layerpositioned between two adjacent first well layers among the first welllayers and having a wider band gap than the band gaps of the first welllayers; the second active layer comprises: a plurality of second welllayers, and a second barrier layer positioned between two adjacentsecond well layers among the second well layers and having a wider bandgap than the band gaps of the second well layers; the second barrierlayer is a nitride semiconductor layer containing an n-type impurity andgallium, and has a higher n-type impurity concentration than an n-typeimpurity concentration of the first barrier layer; and an n-typeimpurity concentration peak in the second barrier layer is located on afirst light emitting part side.
 2. The light emitting element accordingto claim 1, wherein: the second barrier layer comprises, successivelyfrom a first light emitting part side, a first layer and a second layer;the n-type impurity concentration peak in the second barrier layer islocated in the first layer; and a thickness of the first layer is 10% to50% of a thickness of the second barrier layer.
 3. The light emittingelement according to claim 2 wherein: the thickness of the first layeris in a range of 0.5 nm to 2 nm.
 4. The light emitting element accordingto claim 2, wherein: the tunnel junction part comprises a nitridesemiconductor layer containing an n-type impurity; and an n-typeimpurity concentration of the first layer is lower than an n-typeimpurity concentration of the tunnel junction part.
 5. The lightemitting element according to claim 3, wherein: the tunnel junction partcomprises a nitride semiconductor layer containing an n-type impurity;and an n-type impurity concentration of the first layer is lower than ann-type impurity concentration of the tunnel junction part.
 6. The lightemitting element according to claim 2 wherein: an n-type impurityconcentration of the first layer is in a range of 2×10¹⁸ cm⁻³ to 5×10¹⁸cm⁻³.
 7. The light emitting element according to claim 3 wherein: ann-type impurity concentration of the first layer is in a range of 2×10¹⁸cm⁻³ to 5×10¹⁸ cm⁻³.
 8. The light emitting element according to claim 4wherein: the n-type impurity concentration of the first layer is in arange of 2×10¹⁸ cm⁻³ to 5×10¹⁸ cm⁻³.
 9. The light emitting elementaccording to claim 2, wherein: the second barrier layer furthercomprises a third layer that is positioned closer to the first lightemitting part than is the first layer; and an n-type impurityconcentration of the second layer and an n-type impurity concentrationof the third layer are lower than an n-type impurity concentration ofthe first layer.
 10. The light emitting element according to claim 3,wherein: the second barrier layer further comprises a third layer thatis positioned closer to the first light emitting part than is the firstlayer; and an n-type impurity concentration of the second layer and ann-type impurity concentration of the third layer are lower than ann-type impurity concentration of the first layer.
 11. The light emittingelement according to claim 4, wherein: the second barrier layer furtherhas a third layer that is positioned closer to the first light emittingpart than the first layer; and an n-type impurity concentration of thesecond layer and an n-type impurity concentration of the third layer arelower than the n-type impurity concentration of the first layer.
 12. Thelight emitting element according to claim 1, wherein: the second activelayer comprises three or more second well layers and two or more secondbarrier layers; and an n-type impurity concentration peak in each of thesecond barrier layers is located on the first light emitting part side.13. The light emitting element according to claim 2, wherein: the secondactive layer comprises three or more second well layers and two or moresecond barrier layers; and an n-type impurity concentration peak in eachof the second barrier layers is located on the first light emitting partside.
 14. The light emitting element according to claim 3, wherein: thesecond active layer comprises three or more second well layers and twoor more second barrier layers; and an n-type impurity concentration peakin each of the second barrier layers is located on the first lightemitting part side.
 15. The light emitting element according to claim 1,wherein: the second active layer further comprises a third barrier layerat a highest position in the second active layer; and an n-type impurityconcentration of the third barrier layer is lower than the n-typeimpurity concentration of the second barrier layer.
 16. The lightemitting element according to claim 2, wherein: the second active layerfurther comprises a third barrier layer at a highest position in thesecond active layer; and an n-type impurity concentration of the thirdbarrier layer is lower than the n-type impurity concentration of thesecond barrier layer.
 17. The light emitting element according to claim3, wherein: the second active layer further comprises a third barrierlayer at a highest position in the second active layer; and an n-typeimpurity concentration of the third barrier layer is lower than then-type impurity concentration of the second barrier layer.